This invention concerns a clamping device for clamping overlapped edges of metal sheets to be connected by laser welding.
Laser welding of metal sheets is very often used as an alternative to conventional electric welding, for example in the motor vehicle or automotive manufacturing, due to the advantages that laser welding offers in terms of greater speed and reliability, and the lower electrical power required.
The possibilities of using laser welding, however, are limited whenever metal sheets which have been subjected to anticorrosive surface treatment, for example, galvanization, are to be welded, in that, during the welding gases may be generated which give rise to the erosion and perforation of the metal in correspondence with the connection points between the overlapped edges of the metal sheets.
Consequently, in correspondence of the individual welding points there remains an evident trace of the connection between the metal sheets, which does not enable laser welding to be used for joining parts in full view, for example the bodywork of motor vehicles or cars, in that it would jeopardise their aesthetical appearance.
During various tests in the use of laser welding, it was noted that the aforementioned erosion problem of the metal sheets in correspondence of the connection points can be solved by facilitating the dispersion of the gases which are generated during the welding in correspondence with the edges of the metal sheets.
A solution capable of facilitating the dispersion of the harmful gases consists in keeping the overlapped edges of the metal sheets slightly spaced apart from each other, so as to still obtain a laser welding with excellent characteristics in terms of strength and surface finishing, and at the same time obtaining an efficient dispersion of the gases; in particular it was found that an optimum distance xcex4 of the overlapped edges of the metal sheets, in correspondence with the welding points, can range from 5 to 20 hundredths of a millimeter.
The order of magnitude of such values entails the use of a pressure member, or presser, which enables a reciprocal positioning of the overlapped edges of the metal sheets to be set in a very precise and repeatable way.
The pressure devices provided, for example, by the conventional clamping devices currently available, are not capable of guaranteeing such precision and repeatability of the reciprocal positioning of the overlapped edges of the metal sheets, in that the presence of internal clearances in the pressure devices which are added to one another does not allow a sufficiently efficient control of the presser, as to ensure the high degree of precision required for positioning the metal sheets.
Moreover, the progressive wear of the mechanical members of the clamping device tends to increase the clearances inside the device itself, making the positioning of the metal sheets even more precarious as the wear gradually increases.
A possible solution could be the use of an electronic control system for controlling the clamping device, or for operating the presser, of such kind as to automatically compensate the various clearances to enable the correct positioning of the edges of the metal sheets. A solution of this kind, however, would prove to be too complex and difficult to perform, as well as expensive.
An object of this invention is to provide a clamping device for clamping the overlapped edges of metal sheets to be laser welded, which allows a low-cost, reliable, and simple mechanical solution, capable of automatically compensating the clearances of the device, in a precise and repeatable way.
The above can be achieved by means of a device for clamping overlapped edges of metal sheets to be connected by laser welding, comprising:
a box-shaped head having a longitudinal axis;
a clamping arm pivotally connected to the head to rotate between an open condition, a closed condition in which the overlapped edges of the metal sheets are pressed against each other, and an intermediate welding condition in which the overlapped edges of the metal sheets are slightly spaced apart to allow the laser welding;
an electric control actuator; and
a toggle lever mechanism operatively connected between the clamping arm and the electric control actuator, said toggle lever mechanism being connected to the electric control actuator by intermediate transmission members,
characterised by comprising counteracting biasing means for compensating clearances in the operative connection between the clamping arm and the electric control actuator; and
in that said counteracting biasing means are constructed and disposed to rearwardly urge the clamping lever in the closed condition of the clamping mechanism.